linux-kernel - Re: CFS review

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Message-ID: <20070801143624.GA18043@elte.hu>
Date:	Wed, 1 Aug 2007 16:36:24 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Roman Zippel <zippel@...ux-m68k.org>
Cc:	Andi Kleen <andi@...stfloor.org>, Mike Galbraith <efault@....de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: CFS review

* Roman Zippel <zippel@...ux-m68k.org> wrote:

> > jiffies based sched_clock should be soon very rare. It's probably 
> > not worth optimizing for it.
> 
> I'm not so sure about that. sched_clock() has to be fast, so many 
> archs may want to continue to use jiffies. [...]

i think Andi was talking about the vast majority of the systems out 
there. For example, check out the arch demography of current Fedora 
installs (according to the Smolt opt-in UUID based user metrics):

   http://smolt.fedoraproject.org/

   i686:     74743
   x86_64:   18599
   i386:      1208
   ppc:        527
   ppc64:      396
   sparc64:     14
   ---------------
   Total:    95488

even pure i386 (kernels, not systems) is a only 1.2% of all installs. By 
the time the CFS kernel gets into a distro (a few months at minimum, 
typically a year) this percentage will go down further. And embedded 
doesnt really care about task-statistics corner cases [ (it likely 
doesnt have 'top' installed - likely doesnt even have /proc mounted or 
even built in ;-) ].

of course CFS should not do _worse_ stats than what we had before, and 
should not break or massively misbehave. Also, anything sane we can do 
for low-resolution arches we should do (and we already do quite a bit - 
the while wmult stuff is to avoid expensive divisions) - and i regularly 
booted CFS with a low-resolution clock to make sure it works. So i'm not 
trying to duck anything, we've just got to keep our design priorities 
right :-)

	Ingo
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